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If it's true that a lady never reveals her age, then the Sierra
Nevada mountain range, with its rich wilderness and snow-capped
peaks rising above California and Nevada, is quite the lady.
Researchers still don't know exactly how or when those rocky
summits got there.

Now, new research has uncovered a clue in this geologic puzzle.
Using
GPS and space radar technology, scientists found that the
range — which includes Lake Tahoe and the highest peak in the
contiguous United States, the 14,505-foot-tall (4,421 meters)
Mount Whitney — is growing by about a millimeter each year. At
this rate, the entire
Sierra Nevada could have been built in just the last 3
million years, the researchers say.

"There's a surprisingly wide variety of opinions about how and
why the Sierra Nevada goes up, and about the ages and timing of
all the events that contribute to the uplift," said William
Hammond, a geophysicist at the University of Nevada, Reno, who
led the study. "These findings suggest that whatever mechanism is
at play, it's acting on the entire range."

An age-old question

Geologists say there are two possible, and wildly different, ages
for the Sierra Nevada range: either 40 million to 80 million
years old, or only about 3 million years old. [ 50 Amazing Facts About
Earth ]

That's a big difference, Hammond said, because it means the
mountains are either very old and no longer growing, or they're
quite young and still growing at a measurable rate.

To figure out the Sierra Nevada's current growth rate, Hammond's
team combined GPS data with measurements from interferometric
synthetic aperture radar, or "InSAR," a type of space radar.

In this radar technique, a satellite whizzes over the Earth and
uses microwave energy to take snapshots of features like mountain
ranges and earthquake faults. The satellite then revisits the
same spot every month or so to take more microwave snapshots.
Because it can monitor large swaths of landscape over long
periods of time, InSAR data is particularly useful for measuring
slow and steady changes in the shape of the Earth's surface.

"Most of the seismic cycle is made up of periods of time where
the Earth is not shaking, but it is deforming," Hammond told
OurAmazingPlanet. "We're getting better at measuring that slow
shape change as a way of understanding, for example, where the
Earth might break in future earthquakes."

His team analyzed 18 years of Sierra Nevada InSAR images, along
with precise measurements from GPS stations, to zero in on a
growth rate of about 0.04 to 0.08 inches (1-2 millimeters) per
year for the mountains.

This means the entire range, which has an average high elevation
of about 6,500 to 8,200 feet (2,000 to 2,500 meters), could have
been built in less than 3 million years.

How did the mountains get there?

Exactly how the Sierra Nevada range was built is still a mystery,
though, and theories abound.

The mountains lie just west of the
Basin and Range Province in Nevada, where east-west tectonic
forces are in the process of ripping the Earth's crust apart.
Some geologists think this stretching might be causing the
eastern side of the Sierra Nevada to grow upwards.

Another idea comes from seismologists, who believe they see a
weighty blob in the Earth's mantle that may have been
attached to the base of the Sierra Nevada tectonic block. They
think this dense blob used to weigh the block down, like a keel
on the bottom of a ship. Then, sometime between 3 million and 10
million years ago, this keel peeled off and sank down deeper into
the Earth, and the Sierra Nevada block popped up.

Whichever it was, Hammond's team found that the process affected
nearly the entire range — from Lake Tahoe to the Mojave Desert —
and is still building up the Sierra Nevada today.

The team's findings were published on April 27 in the journal
Geology.